NAME

       extend - produce an extended elevation from a .3d file

SYNOPSIS

       extend [ --survey=SURVEY ] [ --specfile=ESPEC_FILE ] [ --show-breaks ] INPUT_3D_FILE [ OUTPUT_3D_FILE ]

DESCRIPTION

              Note:  The  extend program can also work on Compass .plt (as can aven and any other Survex program
              which reads .3d files).

       If no specfile is given, extend starts with the highest station marked as an entrance which has at  least
       one underground survey leg attached to it.  If there are no such stations, the highest deadend station in
       the  survey  (or the highest station if there are no deadends) is used.  Extend puts the first station on
       the left, then folds each leg out individually to the  right,  breaking  loops  arbitrarily  (usually  at
       junctions).

       If  the  output  filename  is  not specified, extend bases the output filename on the input filename, but
       ending  "_extend.3d".   For  example,  extend  deep_pit.3d  produces   an   extended   elevation   called
       deep_pit_extend.3d.

       If  you  pass  --show-breaks  then  a leg flagged as "surface survey" will be added between each point at
       which a loop has been broken - this can be very useful for visualising the result in aven.

       This approach suffices for simple caves or sections of cave, but for more  complicated  situations  human
       intervention is required.  More complex sections of cave can be handled with a specfile giving directions
       to  switch the direction of extension between left and right, to explicitly specify the start station, or
       to break the extension at particular stations or legs.

       The specfile is in a format similar to cavern's data format:

       ;This is a comment

       ; start the elevation at station entrance.a
       *start entrance.a  ;this is a comment after a command

       ; start extending leftwards from station half-way-down.5
       *eleft half-way-down.5

       ; change direction of extension at further-down.8
       *eswap further-down.8

       ; extend right from further-down.junction, but only for
       ; the leg joining it to very-deep.1, other legs continuing
       ; as before
       *eright further-down.junction  very-deep.1

       ; break the survey at station side-loop.4
       *break side-loop.4

       ; break survey at station side-loop.junction but only
       ; for leg going to complex-loop.2
       *break side-loop.junction complex-loop.2

       This approach requires some trial and error, but gives useful results for many caves.  The  most  complex
       systems  would  benefit  from  an  interactive  interface  to  select and view the breaks and switches of
       direction.

SEE ALSO

       aven(1), cavern(1), diffpos(1), dump3d(1), sorterr(1), survexport(1)

                                                24 February 2022                                       EXTEND(1)